As part of a continuing effort to develop aircraft having increased fuel efficiency and decreased weight, many aircraft manufacturers are replacing structures made of aluminum or steel with structure made of composite materials. Composite materials typically have strengths which equal or exceed the strength of a corresponding steel or aluminum structure while weighing significantly less. In addition, composite materials are not as susceptible to corrosion, thereby adding to the life expectancy of the aircraft.
Composite materials achieve their light weight and high strength by the nature of their construction. A composite material is made of graphite fibers that are woven together to form a sheet. Each sheet is then bonded to another sheet forming a layer of a desired thickness. Generally, the fibers of each sheet are oriented in different directions from the fibers of the sheet above and below it. This produces a cross-hatch effect that creates a material much stronger than an individual composite sheet. An epoxy resin is used to bond all the sheets together.
Most aircraft have numerous systems that need to be secured to the aircraft's structure. These systems include fuel pumps, avionics, and many others. The systems are secured to the aircraft structure at points called "details". For composite structures, the details are metallic pads having a shape that generally matches the "footprint" of the system to be secured to the composite structure. The detail must provide a flat, smooth, hard surface at the point where the detail is to be secured in order to ensure a good seal between the system and the structure. In addition, the detail must also provide a good electrical conductive path between the system and the composite structure in order to safely conduct electrical current received in the case of a lightning strike.
During a lightning strike, a large current (up to 200KAmps) enters the aircraft at some point and propagates throughout the aircraft in an attempt to dissipate the electrical energy. One of the most dangerous points for the lightning current to cross is at a junction of dissimilar materials. This is particularly true at the system details where currents flow from the system housings, which are often metal, into the composite aircraft structures. If the conductivity at this junction is low, the large current can vaporize the epoxy resin in the composite material, creating a superheated gas that is expelled with explosive force from under the detail.
In the past, the only way to achieve good conductivity between a detail and a composite structure was to electroplate the detail onto the composite structure. To do this, the composite structure needed to be primed with a conductive paint and then surrounded with a chemical bath. Electric current was then applied to the bath causing a layer of metal to gradually build up on the layer of conductive paint. While the resulting detail did have good conductivity to the underlying composite structure, the process is slow and labor intensive. In addition, the resulting detail was easily peeled away from the composite structure when the system secured to the detail was removed for maintenance or repair. Repair of the detail requires another electroplating process, which is extremely dangerous in aircraft that had previously been fueled unless all fuel vapors have been removed, thereby adding to the cost and delay of repairing a detail.
Therefore, there is a need for a method of creating a detail on a composite aircraft that ensures good conductivity between a composite structure and an aircraft system. In addition, the detail should be strong enough to withstand removal of the system without peeling. Finally, the detail should be able to be repaired in the presence of flammable materials.